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That's weird. Well, please open a terminal and share the output of the following command here. You can copy text to/from the terminal, see its Edit menu. With that information, we can confirm the manual action you should take.

The Software Sources route apparently wasn't working, hence the workaround. As for nomko's command, you may have to replace sudo gedit with gksudo pluma or kdesu kate depending on your desktop environment (former for MATE, latter for KDE). You can also make the needed changes by running this command from the terminal: